The two-cell stage (from about 18 to 38 hours after fertilization) is a critical period of mouse embryo development when a pronounced shift from maternal to embryonic control of development takes place. Recent findings suggest that during this period much of the maternal mRNA is eliminated and is replaced by newly synthesized mRNA derived from the embryonic genome, resulting in a major change in the qualitative pattern of protein synthesis. The proposed research will explore the pattern of change underlying this transition at the level of individual mRNAs. The poly(A)+ RNA population from late two-cell embryos (containing both maternal and embryonic transcripts) will be used for the production of a cDNA library, and a subset of about 200 clones selected at random will be hybridized with 32p-cDNA made to poly(A)+ RNA preparations from embryos at different stages of development from one-cell through blastocyst. The abundance of individual mRNAs corresponding to this set of clones will be quantitatively estimated by comparison to standard clones whose transcript prevalence is known. The results should provide information on the overall developmental changes in mRNA prevalence during early development and the stage-specific regulation of certain mRNA sequences. Studies are proposed also to identify and characterize the RNA associated with so-called "epsilon-particles" (a retrovirus-like structure) which are a prominent feature of the early cleavage stages in mice.